High-strength steel sheet and method for manufacturing same

ABSTRACT

Disclosed is a method comprising: preparing a steel slab with a predetermined chemical composition; subjecting the steel slab to hot rolling by heating it to a temperature of 1100-1300° C., hot rolling it with a finisher delivery temperature of 800-1000° C. to form a hot-rolled steel sheet, and coiling the steel sheet at a mean coiling temperature of 200-500° C.; subjecting the steel sheet to pickling treatment; subjecting the steel sheet to annealing by retaining the steel sheet at a temperature of 740-840° C. for 10-900 s, and the cooling the steel sheet at a mean cooling rate of 5-30° C./s to a cooling stop temperature of 150-350° C.; and subjecting the steel sheet to reheating treatment by reheating the steel sheet to a reheating temperature of higher than 350° C. and 550° C. or lower, and retaining the steel sheet at the reheating temperature for 10 s or more.

TECHNICAL FIELD

This disclosure relates to a high-strength steel sheet with excellent formability which is mainly suitable for automobile structural members and a method for manufacturing the same, and in particular, to provision of a high-strength steel sheet with high productivity that has a tensile strength (TS) of 780 MPa or more and that is excellent in ductility as well as in stretch flangeability and fatigue properties.

BACKGROUND

In order to secure passenger safety upon collision and to improve fuel efficiency by reducing the weight of automotive bodies, high-strength steel sheets reduced in thickness and having a tensile strength (TS) of 780 MPa or more have been increasingly applied to automobile structural members. Further, in recent years, examination has been made of applications of ultra-high-strength steel sheets with 980 MPa and 1180 MPa grade TS.

In general, however, strengthening of steel sheets leads to deterioration in formability. It is thus difficult to achieve both increased strength and excellent formability. Therefore, it is desirable to develop steel sheets with increased strength and excellent formability.

It is also desirable for steel sheets to have excellent fatigue properties since the travelable distance (total running distance) of automobiles depends on the fatigue strength of steel sheets applied to the automobile structural members.

To meet these demands, for example, JP2004218025A (PTL 1) describes “a high-strength steel sheet with excellent workability and shape fixability comprising: a chemical composition containing, in mass %, C: 0.06% to 0.6%, Si+Al: 0.5% to 3%, Mn: 0.5% to 3%, P: 0.15% or less (exclusive of 0%), and S: 0.02% or less (inclusive of 0%); and a structure that contains tempered martensite: 15% or more by area to the entire structure, ferrite: 5% to 60% by area to the entire structure, and retained austenite: 5% or more by volume to the entire structure, and that may contain bainite and/or martensite, wherein a ratio of the retained austenite transforming to martensite upon application of a 2% strain is 20% to 50%.

JP2011195956A (PTL 2) describes “a high-strength thin steel sheet with excellent elongation and hole expansion formability, comprising: a chemical composition containing, in mass %, C: 0.05% or more and 0.35% or less, Si: 0.05% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.0010% or more and 0.1% or less, S: 0.0005% or more and 0.05% or less, N: 0.0010% or more and 0.010% or less, and Al: 0.01% or more and 2.0% or less, and the balance consisting of iron and incidental impurities; and a metallographic structure that includes a dominant phase of ferrite, bainite, or tempered martensite, and a retained austenite phase in an amount of 3% or more and 30% or less, wherein at a phase interface at which the austenite phase comes in contact with the ferrite phase, bainite phase, and martensite phase, a mean carbon concentration in the austenite phase is 0.6% or more and 1.2% or less, and austenite grains that satisfy Cgb/Cgc>1.3 are present in the austenite phase in an amount of 50% or more, where Cgc is a central carbon concentration and Cgb is a carbon concentration at grain boundaries of austenite grains.

JP201090475A (PTL 3) describes “a high-strength steel sheet comprising a chemical composition containing, in mass %, C: 0.17% or more and 0.73% or less, Si: 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less, where Si+Al is 0.7% or more, and the balance consisting of Fe and incidental impurities; and a structure that contains martensite: 10% or more and 90% or less by area to the entire steel sheet structure, retained austenite content: 5% or more and 50% or less, and bainitic ferrite in upper bainite: 5% or more by area to the entire steel sheet structure, wherein the steel sheet satisfies conditions that 25% or more of the martensite is tempered martensite, a total of the area ratio of the martensite to the entire steel sheet structure, the retained austenite content, and the area ratio of the bainitic ferrite in upper bainite to the entire steel sheet structure is 65% or more, and an area ratio of polygonal ferrite to the entire steel sheet structure is 10% or less (inclusive of 0%), and wherein the steel sheet has a mean carbon concentration of 0.70% or more in the retained austenite and has a tensile strength of 980 MPa or more.

JP2008174802A (PTL 4) describes “a high-strength cold-rolled steel sheet with a high yield ratio and having a tensile strength of 980 MPa or more, the steel sheet comprising, on average, a chemical composition that contains, by mass %, C: more than 0.06% and 0.24% or less, Si≦0.3%, Mn: 0.5% to 2.0%, P≦0.06%, S≦0.005%, Al≦0.06%, N≦0.006%, Mo: 0.05% to 0.5%, Ti: 0.03% to 0.2%, and V: more than 0.15% and 1.2% or less, and the balance consisting of Fe and incidental impurities, wherein the contents of C, Ti, Mo, and V satisfy 0.8≦(C/12)/{(Ti/48)+(Mo/96)+(V/51)}≦1.5, and wherein an area ratio of ferrite phase is 95% or more, and carbides containing Ti, Mo, and V with a mean grain size of less than 10 nm are diffused and precipitated, where Ti, Mo, and V contents represented by atomic percentage satisfy V/(Ti+Mo+V)≧0.3.

JP2010275627A (PTL 5) describes “a high-strength steel sheet with excellent workability comprising a chemical composition containing C: 0.05 mass % to 0.3 mass %, Si: 0.01 mass % to 2.5 mass %, Mn: 0.5 mass % to 3.5 mass %, P: 0.003 mass % to 0.100 mass %, S: 0.02 mass % or less, and Al: 0.010 mass % to 1.5 mass %, where a total of the Si and Al contents is 0.5 mass % to 3.0 mass %, and the balance consisting of Fe and incidental impurities; and a metallic structure that contains, by area, ferrite: 20% or more, tempered martensite: 10% to 60%, and martensite: 0% to 10%, and that contains, by volume, retained austenite: 3% to 10%, where a ratio (m)/(f) of a Vickers hardness (m) of the tempered martensite to a Vickers hardness (f) of the ferrite is 3.0 or less.

JP4268079B (PTL 6) describes “an ultra-high-strength steel sheet exhibiting an excellent elongation in an ultra-high-strength range with a tensile strength of 1180 MPa or more, and having excellent hydrogen embrittlement resistance, the steel sheet comprising a chemical composition containing, in mass %, C: 0.06% to 0.6%, Si+Al: 0.5% to 3%, Mn: 0.5% to 3%, P: 0.15% or less (exclusive of 0%), S: 0.02% or less (inclusive of 0%), and the balance: Fe and incidental impurities; and a structure that contains tempered martensite: 15% to 60% by area to the entire structure, ferrite: 5% to 50% by area to the entire structure, retained austenite: 5% or more by area to the entire structure, and massive martensite with an aspect ratio of 3 or less: 15% to 45%, where an area ratio of fine martensite having a mean grain size of 5 μm or less in the massive martensite is 30% or more.

PTL 6 also describes a method for manufacturing the ultra-high-strength steel sheet comprising: heating and retaining a steel satisfying the aforementioned composition at a temperature from A₃ to 1100° C. for 10 s or more, and then cooling the steel at a mean cooling rate of 30° C./s or higher to a temperature at or below Ms, and repeating this cycle at least twice; and heating and retaining the steel at a temperature from (A₃-25° C.) to A₃ for 120 s to 600 s, and then cooling the steel at a mean cooling rate of 3° C./s or higher to a temperature at or above Ms and at or below Bs, at which the steel is retained for at least one second.

CITATION LIST Patent Literature

PTL 1: JP2004218025A

PTL 2: JP2011195956A

PTL 3: JP201090475A

PTL 4: JP2008174802A

PTL 5: JP2010275627A

PTL 6: JP4268079B

SUMMARY Technical Problem

In fact, PTL 1 teaches the high-strength steel sheet has excellent workability and shape fixability, PTL 2 teaches the high-strength thin steel sheet has excellent elongation and hole expansion formability, PTL 3 teaches the high-strength steel sheet has excellent workability, in particular, excellent ductility and stretch flangeability. None of them however takes into account fatigue properties.

The high-strength cold-rolled steel sheet with a high yield ratio described in PTL 4 uses expensive elements, Mo and V, which results in increased costs and a low elongation (EL), which is as low as approximately 19%.

The high-strength steel sheet described in PTL 5 exhibits, for example, TS of 980 MPa or more and TS×EL of approximately 24000 MPa·%, which remain, although may be relatively high when compared to general-use material, insufficient to meet the ongoing requirements for steel sheets.

The ultra-high tensile-strength steel sheet described in PTL 6 requires performing annealing treatment at least three times during its manufacture, resulting in low productivity in actual facilities.

It could thus be helpful to provide a method that can manufacture a high-strength steel sheet with high productivity that has a tensile strength (TS) of 780 MPa or more and that is excellent not only in ductility but also in stretch flangeability and fatigue properties, by performing a single annealing treatment at a ferrite-austenite dual phase region to form a fine structure that contains appropriate amounts of ferrite, bainitic ferrite, and retained austenite, and performing reheating following the annealing treatment so that an appropriate amount of tempered martensite is present in the structure.

It could also be helpful to provide a high-strength steel sheet manufactured by the method. As used herein, the term “high-strength steel sheet” is intended to include high-strength galvanized steel sheets having a galvanized surface.

A steel sheet obtained according to the disclosure has the following target properties:

Tensile strength (TS)

-   -   780 MPa or more

Ductility

-   -   TS 780 MPa grade: EL≧34%     -   TS 980 MPa grade: EL≧27%     -   TS 1180 MPa grade: EL≧23%

Balance between strength and ductility

-   -   TS×EL≧27000 MPa·%

Stretch flangeability

-   -   TS 780 MPa grade: λ≧40%     -   TS 980 MPa grade: λ≧30%     -   TS 1180 MPa grade: λ≧20%     -   The maximum hole expansion ratio λ (%)={(D_(f)−D₀)/D₀}×100,         where D_(f) is the hole diameter (mm) upon cracking and D₀ is         the initial hole diameter (mm).

Fatigue property

-   -   fatigue limit strength ≧400 MPa, and fatigue ratio ≧0.40     -   As used herein, the term “fatigue ratio” means a ratio of         fatigue limit strength to tensile strength.

Solution to Problem

Upon carefully examining how to manufacture a steel sheet having TS of 780 MPa or more and excellent in ductility, stretch flangeability, and fatigue properties with high productivity, we discovered the following.

-   -   (1) To obtain a steel sheet having a tensile strength (TS) of         780 MPa or more and excellent in ductility, stretch         flangeability, and fatigue properties, it is important to         prepare an appropriate chemical composition and to form a         structure that contains appropriate amounts of ferrite, bainitic         ferrite, and retained austenite, and in which fine retained         austenite and fine bainitic ferrite are distributed.     -   (2) In addition, to form such a structure, it is important to         provide the steel sheet with a structure prior to annealing         treatment in which a single phase structure of martensite, a         single phase structure of bainite, or a martensite-bainite mixed         structure is dominantly present, while controlling annealing         treatment conditions properly.     -    In this respect, in order for the steel sheet to have such a         pre-annealing structure without subjection to separate annealing         treatment, it is important to perform appropriate slab reheating         and optimize hot rolling conditions, in particular, to keep the         mean coiling temperature (CT) following hot rolling low.     -   (3) Moreover, when cold rolling is performed after hot rolling,         it is important to set a low rolling reduction such that the         resulting structure of the hot-rolled steel sheet in which a         single phase structure of martensite, a single phase structure         of bainite, or a martensite-bainite mixed phase structure is         dominantly present will remain intact as much as possible.     -   (4) Additionally, to improve stretch flangeability, it is         important for the structure to contain an appropriate amount of         tempered martensite and, to this end, it is of importance to         keep the cooling stop temperature after annealing low and         perform subsequent reheating treatment under proper conditions.         The disclosure is based on the aforementioned discoveries and         further studies.

Specifically, the primary features of this disclosure are as described below.

1. A method for manufacturing a high-strength steel sheet, the method comprising: preparing a steel slab containing (consisting of), in mass %, C: 0.10% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and the balance consisting of Fe and incidental impurities; subjecting the steel slab to hot rolling by heating the steel slab to a temperature of 1100° C. or higher and 1300° C. or lower, hot rolling the steel slab with a finisher delivery temperature of 800° C. or higher and 1000° C. or lower to form a hot-rolled steel sheet, and coiling the hot-rolled steel sheet at a mean coiling temperature of 200° C. or higher and 500° C. or lower; subjecting the hot-rolled steel sheet to pickling treatment; subjecting the hot-rolled steel sheet to annealing by retaining the hot-rolled steel sheet at a temperature of 740° C. or higher and 840° C. or lower for 10 s or more and 900 s or less, and then cooling the hot-rolled steel sheet at a mean cooling rate of 5° C./s or higher and 30° C./s or lower to a cooling stop temperature of 150° C. or higher and 350° C. or lower; and subjecting the hot-rolled steel sheet to reheating treatment by reheating the hot-rolled steel sheet to a reheating temperature of higher than 350° C. and 550° C. or lower, and retaining the hot-rolled steel sheet at the reheating temperature for 10 s or more.

2. The method for manufacturing a high-strength steel sheet according to 1., the method further comprising prior to the annealing, cold rolling the hot-rolled steel sheet at a rolling reduction of less than 30% to form a cold-rolled steel sheet, wherein in the annealing, the cold-rolled steel sheet is retained at a temperature of 740° C. or higher and 840° C. or lower for 10 s or more and 900 s or less, and cooled at a mean cooling rate of 5° C./s or higher and 30° C./s or lower to a cooling stop temperature of 150° C. or higher and 350° C. or lower, and in the reheating treatment, the cold-rolled steel sheet is reheated to a reheating temperature of higher than 350° C. and 550° C. or lower and retained at the reheating temperature for 10 s or more.

3. The method for manufacturing a high-strength steel sheet according to 1. or 2., the method further comprising after the reheating treatment, subjecting the hot-rolled steel sheet or the cold-rolled steel sheet to galvanizing treatment.

4. The method for manufacturing a high-strength steel sheet according to any of 1. to 3., wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.

5. The method for manufacturing a high-strength steel sheet according to any of 1. to 4., wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.

6. A high-strength steel sheet comprising: a steel chemical composition containing (consisting of), in mass %, C: 0.10% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and the balance consisting of Fe and incidental impurities; and a steel structure that contains a total of 30% or more and 75% or less by area of ferrite and bainitic ferrite, 5% or more and 15% or less by area of tempered martensite, and 8% or more by volume of retained austenite, wherein the retained austenite has a mean grain size of 2 μm or less and the bainitic ferrite has a mean free path of 3 μm or less.

7. The high-strength steel sheet according to 6., wherein the steel chemical composition further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.

8. The high-strength steel sheet according to 6. or 7., wherein the steel chemical composition further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.

Advantageous Effect

According to the disclosure, it becomes possible to manufacture a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, stretch flangeability, and fatigue properties with high productivity.

Also, a high-strength steel sheet manufactured by the method according to the disclosure is highly beneficial in industrial terms, because it can improve fuel efficiency when applied to, e.g., automobile structural members by a reduction in the weight of automotive bodies.

DETAILED DESCRIPTION

The present invention will be specifically described below.

According to the method disclosed herein, a steel slab with a predetermined chemical composition is heated and hot rolled. At this point, it is important to keep the mean coiling temperature (CT) during hot rolling low so that the hot-rolled steel sheet is provided with a structure in which a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present. It is also important when cold rolling is performed after hot rolling to set as low a rolling reduction as possible so that the resulting structure of the hot-rolled steel sheet will remain intact as much as possible.

In this way, a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present in the structure of the steel sheet before subjection to annealing treatment. Consequently, even when annealing treatment is performed just once at a ferrite-austenite dual phase region, it becomes possible to form a structure that contains appropriate amounts of ferrite, bainitic ferrite, and retained austenite, and in which fine retained austenite and fine bainitic ferrite are distributed.

In addition, by causing the cooling stop temperature after annealing to drop to 350° C. or lower and performing reheating treatment under proper conditions, the structure may contain an appropriate amount of tempered martensite. As a result, it becomes possible to manufacture a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, stretch flangeability, and fatigue properties with high productivity.

Firstly, the reasons for the limitations on the chemical composition of the steel manufactured according to our methods are described.

When components are expressed in “%,” this refers to “mass %” unless otherwise specified.

C: 0.10% or More and 0.35% or Less

C is an element that is important for increasing the strength of steel, has a high solid solution strengthening ability, and is essential for guaranteeing the presence of a desired amount of retained austenite to improve ductility. If the C content is below 0.10%, it becomes difficult to obtain the required amount of retained austenite. If the C content exceeds 0.35%, however, the steel sheet is made brittle or susceptible to delayed fracture. Therefore, the C content is 0.10% or more and 0.35% or less, preferably 0.15% or more and 0.30% or less, and more preferably 0.18% or more and 0.26% or less.

Si: 0.50% or More and 2.50% or Less

Si is an element that is effective in suppressing decomposition of retained austenite to carbides. Si also exhibits a high solid solution strengthening ability in ferrite, and has the property of purifying ferrite by facilitating solute C diffusion from ferrite to austenite to improve ductility. Moreover, Si dissolved in ferrite improves strain hardenability and increases the ductility of ferrite itself. To obtain this effect, the Si content needs to be 0.50% or more. If the Si content exceeds 2.50%, however, an abnormal structure grows, causing ductility to deteriorate. Therefore, the Si content is 0.50% or more and 2.50% or less, preferably 0.80% or more and 2.00% or less, and more preferably 1.20% or more and 1.80% or less.

Mn: 2.00% or More and Less than 3.50%

Mn is effective in guaranteeing strength. Mn also improves hardenability to facilitate formation of a multi-phase structure. Moreover, Mn acts to suppress formation of ferrite and pearlite during a cooling process after hot rolling, and thus is an effective element in causing the hot-rolled sheet to have a structure in which a low temperature transformation phase (bainite or martensite) is dominantly present. To obtain this effect, the Mn content needs to be 2.00% or more. If the Mn content is 3.50% or more, however, Mn segregation becomes significant in the sheet thickness direction, leading to deterioration of fatigue properties. Therefore, the Mn content is 2.00% or more and less than 3.50%, preferably 2.00% or more and 3.00% or less, and more preferably 2.00% or more and 2.80% or less.

P: 0.001% or More and 0.100% or Less

P is an element that has a solid solution strengthening effect and can be added depending on a desired strength. P also facilitates transformation to ferrite, and thus is an effective element in forming a multi-phase structure. To obtain this effect, the P content needs to be 0.001% or more. If the P content exceeds 0.100%, however, weldability degrades and, when a galvanized layer is subjected to alloying treatment, the alloying rate decreases, impairing galvanizing quality. Therefore, the P content is 0.001% or more and 0.100% or less, and preferably 0.005% or more and 0.050% or less.

S: 0.0001% or More and 0.0200% or Less

S segregates to grain boundaries, makes the steel brittle during hot working, and forms sulfides to reduce local deformability. Therefore, the S content needs to be 0.0200% or less. Under manufacturing constraints, however, the S content is necessarily 0.0001% or more. Therefore, the S content is 0.0001% or more and 0.0200% or less, and preferably 0.0001% or more and 0.0050% or less.

N: 0.0005% or More and 0.0100% or Less

N is an element that deteriorates the anti-aging property of steel. Deterioration of the anti-aging property becomes more pronounced, particularly when the N content exceeds 0.0100%. Under manufacturing constraints, the N content is necessarily 0.0005% or more, although smaller N contents are more preferable. Therefore, the N content is 0.0005% or more and 0.0100% or less, and preferably 0.0005% or more and 0.0070% or less.

In addition to the above basic components, at least one element selected from the group consisting of Ti and B may also be included. In particular, when the steel contains both Ti and B in appropriate amounts, the resulting hot-rolled sheet may be provided more advantageously with a structure in which a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present.

Ti: 0.005% or More and 0.100% or Less

Ti forms fine precipitates during hot rolling or annealing to increase strength. In addition, Ti precipitates as TiN with N, and may thus suppress precipitation of BN when B is added to the steel, thereby effectively bringing out the effect of B as described below. To obtain this effect, the Ti content needs to be 0.005% or more. If the Ti content exceeds 0.100%, however, strengthening by precipitation works excessively, leading to deterioration of ductility. Therefore, the Ti content is preferably 0.005% or more and 0.100% or less, and more preferably 0.010% or more and 0.080% or less.

B: 0.0001% or More and 0.0050% or Less

B has the effect of suppressing ferrite-pearlite transformation during a cooling process after hot rolling so that the hot-rolled sheet has a structure in which a low temperature transformation phase (bainite or martensite), in particular martensite is dominantly present. B is also effective in increasing the strength of steel. To obtain this effect, the B content needs to be 0.0001% or more. However, excessively adding B beyond 0.0050% forms excessive martensite, raising a concern that ductility might decrease due to a rise in strength. Therefore, the B content is preferably 0.0001% or more and 0.0050% or less, and more preferably 0.0005% or more and 0.0030% or less.

Mn Content/B Content: 2100 or Less

In particular for a low-Mn chemical composition, ferrite-pearlite transformation develops during a cooling process after hot rolling, which tends to cause ferrite and/or pearlite to be present in the structure of the hot-rolled sheet. As such, to bring out the above-described addition effect of B sufficiently, it is preferred that the Mn content divided by the B content (Mn content/B content) equals 2100 or less, and more preferably 2000 or less. No lower limit is particularly placed on the Mn content/B content, yet a preferred lower limit is approximately 300.

In addition to the above components, at least one element selected from the group consisting of the following may also be included:

Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.

Al: 0.01% or More and 1.00% or Less

Al is an element that is effective in forming ferrite and improving the balance between strength and ductility. To obtain this effect, the Al content needs to be 0.01% or more. On the other hand, an Al content exceeding 1.00% leads to deterioration of surface characteristics. Therefore, when Al is added to steel, the Al content is 0.01% or more and 1.00% or less, and preferably 0.03% or more and 0.50% or less.

Nb: 0.005% or More and 0.100% or Less

Nb forms fine precipitates during hot rolling or annealing to increase strength. To obtain this effect, the Nb content needs to be 0.005% or more. If the Nb content exceeds 0.100%, however, formability deteriorates. Therefore, when Nb is added to steel, the Nb content is 0.005% or more and 0.100% or less.

Cr: 0.05% or More and 1.00% or Less, Cu: 0.05% or More and 1.00% or Less

Cr and Cu not only serve as solid-solution-strengthening elements, but also act to stabilize austenite in a cooling process during annealing, facilitating formation of a multi-phase structure. To obtain this effect, the Cr and Cu contents each need to be 0.05% or more. If the Cr and Cu contents both exceed 1.00%, formability deteriorates. Therefore, when Cr and Cu are added to steel, respective contents are 0.05% or more and 1.00% or less.

Sb: 0.002% or More and 0.200% or Less, Sn: 0.002% or More and 0.200% or Less

Sb and Sn may be added as necessary for suppressing decarbonization of a region extending from the surface layer of the steel sheet to a depth of about several tens of micrometers, which is caused by nitriding and/or oxidation of the steel sheet surface. Suppressing such nitriding or oxidation is effective in preventing a reduction in the amount of martensite formed in the steel sheet surface and guaranteeing strength. To obtain this effect, the Sb and Sn contents each need to be 0.002% or more. However, excessively adding any of these elements beyond 0.200% leads to deterioration of toughness. Therefore, when Sb and Sn are added to steel, respective contents are 0.002% or more and 0.200% or less.

Ta: 0.001% or More and 0.100% or Less

As is the case with Ti and Nb, Ta forms alloy carbides or alloy carbonitrides, and contributes to increasing the strength of steel. It is also believed that Ta has the effect of significantly suppressing coarsening of precipitates when partially dissolved in Nb carbides or Nb carbonitrides to form complex precipitates, such as (Nb, Ta) (C, N), and providing a stable contribution to increasing strength through strengthening by precipitation. This precipitate-stabilizing effect can be obtained when the Ta content is 0.001% or more. However, excessively adding Ta beyond 0.100% fails to further increase the precipitate-stabilizing effect, but instead increases alloy costs. Therefore, when Ta is added to steel, the Ta content is 0.001% or more and 0.100% or less.

Ca: 0.0005% or More and 0.0050% or Less, Mg: 0.0005% or More and 0.0050% or Less, REM: 0.0005% or More and 0.0050% or Less

Ca, Mg, and REM are elements that are used for deoxidation, and are effective in causing spheroidization of sulfides and mitigating the adverse effect of sulfides on local ductility and stretch flangeability. To obtain this effect, Ca, Mg, and REM each need to be added to steel in an amount of 0.0005% or more. However, excessively adding Ca, Mg, and REM beyond 0.0050% leads to increased inclusions and the like, causing defects on the steel sheet surface and internal defects. Therefore, when Ca, Mg, and REM are added to steel, respective contents are 0.0005% or more and 0.0050% or less.

The balance other than the above components consists of Fe and incidental impurities.

The following provides a description of manufacturing conditions in the method according to the disclosure.

The method for manufacturing a high-strength steel sheet according to the disclosure comprises: preparing a steel slab with the aforementioned chemical composition; subjecting the steel slab to hot rolling by heating the steel slab to a temperature of 1100° C. or higher and 1300° C. or lower, hot rolling the steel slab with a finisher delivery temperature of 800° C. or higher and 1000° C. or lower to form a hot-rolled steel sheet, and coiling the hot-rolled steel sheet at a mean coiling temperature of 200° C. or higher and 500° C. or lower; subjecting the hot-rolled steel sheet to pickling treatment; optionally cold rolling the hot-rolled steel sheet at a rolling reduction below 30% to form a cold-rolled steel sheet; subjecting the hot-rolled or cold-rolled steel sheet to annealing by retaining the steel sheet at a temperature of 740° C. or higher and 840° C. or lower for 10 s or more and 900 s or less, and then cooling the steel sheet at a mean cooling rate of 5° C./s or higher and 30° C./s or lower to a cooling stop temperature of 150° C. or higher and 350° C. or lower; and subsequently subjecting the hot-rolled or cold-rolled steel sheet to reheating treatment by reheating the steel sheet to a reheating temperature of higher than 350° C. and 550° C. or lower, and retaining the steel sheet at the reheating temperature for 10 s or more. In the above steps, the temperatures, such as the finisher delivery temperature, the mean coiling temperature, and the like, all represent temperatures measured at the steel sheet surface. The mean cooling rate is also calculated from temperatures measured at the steel sheet surface. The following explains the reasons for the limitations placed on the manufacturing conditions.

Steel Slab Heating Temperature: 1100° C. or Higher and 1300° C. or Lower

Precipitates that are present at the time of heating of a steel slab will remain as coarse precipitates in the resulting steel sheet, making no contribution to strength. Thus, remelting of any Ti- and Nb-based precipitates precipitated during casting is required. In this respect, if a steel slab is heated at a temperature below 1100° C., it is difficult to cause sufficient melting of carbides, leading to problems such as an increased risk of trouble during hot rolling resulting from increased rolling load. In addition, for obtaining a smooth steel sheet surface, it is necessary to scale-off defects on the surface layer of the slab, such as blow hole generation, segregation, and the like, and to reduce cracks and irregularities on the steel sheet surface. Therefore, the steel slab heating temperature needs to be 1100° C. or higher. If the steel slab heating temperature exceeds 1300° C., however, scale loss increases as oxidation progresses. Therefore, the steel slab heating temperature needs to be 1300° C. or lower. For this reason, the steel slab heating temperature is 1100° C. or higher and 1300° C. or lower, and preferably 1150° C. or higher and 1250° C. or lower.

A steel slab is preferably made with continuous casting to prevent macro segregation, yet may be produced with other methods such as ingot casting or thin slab casting. The steel slab thus produced may be cooled to room temperature and then heated again according to the conventional method. Alternatively, there can be employed without problems what is called “energy-saving” processes, such as hot direct rolling or direct rolling in which either a warm steel slab without being fully cooled to room temperature is charged into a heating furnace, or a steel slab undergoes heat retaining for a short period and immediately hot rolled. Further, a steel slab is subjected to rough rolling under normal conditions and formed into a sheet bar. When the heating temperature is low, the sheet bar is preferably heated using a bar heater or the like prior to finish rolling from the viewpoint of preventing troubles during hot rolling.

Finisher Delivery Temperature in Hot Rolling: 800° C. or Higher and 1000° C. or Lower

The heated steel slab is hot rolled through rough rolling and finish rolling to form a hot-rolled steel sheet. At this point, when the finisher delivery temperature exceeds 1000° C., the amount of oxides (scales) generated suddenly increases and the interface between the steel substrate and oxides becomes rough, which tends to impair the surface quality after pickling and cold rolling. In addition, any hot-rolling scales remaining after pickling adversely affect ductility. Further, grain size increases excessively and fatigue properties deteriorate. On the other hand, if the finisher delivery temperature is below 800° C., rolling load and burden increase, rolling is performed more often in a state in which recrystallization of austenite does not occur, and an abnormal texture develops. As a result, the final product has a significant planar anisotropy, and not only does the material properties become less uniform, but also the ductility itself deteriorate. Therefore, the finisher delivery temperature in hot rolling needs to be 800° C. or higher and 1000° C. or lower, and preferably 820° C. or higher and 950° C. or lower.

Mean Coiling Temperature after Hot Rolling: 200° C. or Higher and 500° C. or Lower

Setting of mean coiling temperature after the hot rolling is very important for the method according to the disclosure. Specifically, when the mean coiling temperature after the hot rolling is above 500° C., ferrite and pearlite form during cooling and retaining processes after the hot rolling. Consequently, it becomes difficult to provide the hot-rolled sheet with a structure in which a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present, making it difficult to impart a desired ductility to the steel sheet obtained after annealing or to balance its strength and ductility. If the mean coiling temperature after the hot rolling is below 200° C., the hot-rolled steel sheet is degraded in terms of shape, deteriorating productivity. Therefore, the mean coiling temperature after the hot rolling needs to be 200° C. or higher and 500° C. or lower, preferably 300° C. or higher and 450° C. or lower, and more preferably 350° C. or higher and 450° C. or lower.

Finish rolling may be performed continuously by joining rough-rolled sheets during the hot rolling. Rough-rolled sheets may be coiled on a temporary basis. At least part of finish rolling may be conducted as lubrication rolling to reduce rolling load in hot rolling. Conducting lubrication rolling in such a manner is effective from the perspective of making the shape and material properties of a steel sheet uniform. In lubrication rolling, the coefficient of friction is preferably 0.10 or more and 0.25 or less.

The hot-rolled steel sheet thus produced is subjected to pickling. Pickling enables removal of oxides from the steel sheet surface, and is thus important to ensure that the high-strength steel sheet as the final product has good chemical convertibility and a sufficient quality of coating. Pickling may be performed in one or more batches.

Rolling Reduction in Cold Rolling: Less than 30%

Additionally, the hot-rolled steel sheet may be subjected to cold rolling to form a cold-rolled steel sheet. When cold rolling is performed, rolling reduction in cold rolling is of great importance. Specifically, if the rolling reduction is 30% or more, a low temperature transformation phase is broken in the structure of the hot-rolled sheet. Consequently, it becomes difficult to provide the steel sheet obtained after the annealing with a structure that contains appropriate amounts of ferrite, bainitic ferrite, and retained austenite, and in which fine retained austenite and fine bainitic ferrite are distributed, making it difficult to ensure ductility, balance strength and ductility, or guarantee good fatigue properties. Therefore, the rolling reduction in cold rolling is less than 30%, preferably 25% or less, and more preferably 20% or less. No lower limit is particularly placed on the rolling reduction in cold rolling. It may be greater than 0%. The number of rolling passes and the rolling reduction per pass are not particularly limited, and the effect of the disclosure may be obtained with any number of rolling passes and any rolling reduction per pass.

Annealing Temperature: 740° C. or Higher and 840° C. or Lower

An annealing temperature below 740° C. cannot ensure formation of a sufficient amount of austenite during the annealing. Consequently, a desired amount of retained austenite cannot be obtained in the end, making it difficult to yield good ductility and to balance strength and ductility. On the other hand, an annealing temperature above 840° C. is within a temperature range of austenite single phase, and a desired amount of fine retained austenite cannot be produced in the end, which makes it difficult again to ensure good ductility and to balance strength and ductility. Therefore, the annealing temperature is 740° C. or higher and 840° C. or lower, and preferably 750° C. or higher and 830° C. or lower.

Annealing Treatment Holding Time: 10 s or More and 900 s or Less

A annealing treatment holding time shorter than 10 s cannot ensure formation of a sufficient amount of austenite during the annealing. Consequently, a desired amount of retained austenite cannot be obtained in the end, making it difficult to yield good ductility and to balance strength and ductility. On the other hand, an annealing treatment holding time longer than 900 s causes grain coarsening, a desired amount of fine retained austenite cannot be produced in the end, making it difficult to ensure good ductility and to balance strength and ductility. This also inhibits productivity. Therefore, the annealing treatment holding time is 10 s or more and 900 s or less, preferably 30 s or more and 750 s or less, and more preferably 60 s or more and 600 s or less.

Mean Cooling Rate to a Cooling Stop Temperature of 150° C. or Higher and 350° C. or Lower: 5° C./s or Higher and 30° C./s or Lower

If the mean cooling rate to a cooling stop temperature of 150° C. or higher and 350° C. or lower is below 5° C./s, a large amount of ferrite is produced during cooling, making it difficult to guarantee a desired strength. On the other hand, if the mean cooling rate is above 30° C./s, a low temperature transformation phase forms excessively, degrading ductility. Therefore, the mean cooling rate to a cooling stop temperature of 150° C. or higher and 350° C. or lower is 5° C./s or higher and 30° C./s or lower, and preferably 10° C./s or higher and 30° C./s or lower. The cooling in the annealing is preferably performed by gas cooling; however, furnace cooling, mist cooling, roll cooling, water cooling, and the like can also be employed in combination.

In addition, if the cooling stop temperature is above 350° C., it is higher than the martensite transformation starting temperature (Ms), with the result that tempered martensite is not produced when reheating treatment is performed subsequently, hard and fresh martensite (martensite not tempered) remains in the resulting structure, and hole expansion formability (stretch flangeability) ends up deteriorating. On the other hand, if the cooling stop temperature is below 150° C., austenite transforms to martensite in a large amount, and a desired amount of retained austenite cannot be obtained in the end, making it difficult to obtain good ductility and to balance strength and ductility.

Therefore, the cooling stop temperature is 150° C. or higher and 350° C. or lower, and preferably 180° C. or higher and 320° C. or lower.

Reheating Temperature: Higher than 350° C. and 550° C. or Lower

If the reheating temperature is above 550° C., decomposition of retained austenite occurs, and a desired amount of retained austenite cannot be obtained in the end, making it difficult to yield good ductility and balance strength and ductility. On the other hand, if the heating temperature is 350° C. or lower, a desired amount of tempered martensite cannot be obtained, making it difficult to ensure hole expansion formability (stretch flangeability). Therefore, the reheating temperature is higher than 350° C. and 550° C. or lower, and preferably 370° C. or higher and 530° C. or lower.

Holding Time at Reheating Temperature: 10 s or More

If the holding time at the reheating temperature is shorter than 10 s, there is insufficient time for the concentration of C (carbon) into austenite to progress, making it difficult to ensure a desired amount of retained austenite in the end. Therefore, the holding time at the reheating temperature is 10 s or more. However, a holding time longer than 600 s does not increase the amount of retained austenite and ductility does not significantly improve, where the effect reaches a plateau. Therefore, the holding time at the reheating temperature is preferably 600 s or less, more preferably 30 s or more and 500 s or less, and still more preferably 60 s or more and 400 s or less. Cooling after the holding is not particularly limited, and any method may be used to implement cooling to a desired temperature.

The steel sheet thus obtained may be subjected to galvanizing treatment such as hot-dip galvanizing.

For example, when hot-dip galvanizing is performed, the above-described steel sheet subjected to the annealing treatment is immersed in a galvanizing bath at 440° C. or higher and 500° C. or lower for hot-dip galvanizing, after which coating weight adjustment is performed using gas wiping or the like. For hot-dip galvanizing, a galvanizing bath with an Al content of 0.10% or more and 0.22% or less is preferably used. When a galvanized layer is subjected to alloying treatment, the alloying treatment is performed in a temperature range of 470° C. to 600° C. after hot-dip galvanizing. If alloying treatment is performed at a temperature above 600° C., untransformed austenite transforms to pearlite, where the presence of a desired volume fraction of retained austenite cannot be ensured and ductility may degrade. Therefore, when a galvanized layer is subjected to alloying treatment, the alloying treatment is preferably performed in a temperature range of 470° C. to 600° C. Electrogalvanized plating may also be performed.

Moreover, when skin pass rolling is performed after the heat treatment, the skin pass rolling is preferably performed with a rolling reduction of 0.1% or more and 1.0% or less. A rolling reduction below 0.1% provides only a small effect and complicates control, and hence 0.1% is the lower limit of the favorable range. On the other hand, a rolling reduction above 1.0% significantly degrades productivity, and thus 1.0% is the upper limit of the favorable range.

The skin pass rolling may be performed on-line or off-line. Skin pass may be performed in one or more batches with a target rolling reduction. No particular limitations are placed on other manufacturing conditions, yet from the perspective of productivity, the aforementioned series of processes such as annealing, hot-dip galvanizing, and alloying treatment on a galvanized layer are preferably carried out on a CGL (Continuous Galvanizing Line) as the hot-dip galvanizing line. After the hot-dip galvanizing, wiping may be performed for adjusting the coating amounts.

The following describes the microstructure of a steel sheet manufactured by the method according to the disclosure.

Total Area Ratio of Ferrite and Bainitic Ferrite: 30% or More and 75% or Less

A high-strength steel sheet manufactured by the method according to the disclosure comprises a multi-phase structure in which retained austenite having an influence mainly on ductility and, more preferably, a small amount of martensite affecting strength are diffused in a structure in which soft ferrite with high ductility is dominantly present. In addition, bainitic ferrite forms adjacent to ferrite and retained austenite/martensite, and reduces the difference in hardness between ferrite and retained austenite and between ferrite and martensite to suppress the occurrence of cracking during hole expansion test and of fatigue cracking during fatigue test. To ensure sufficient ductility, the total area ratio of ferrite and bainitic ferrite needs to be 30% or more. On the other hand, to secure strength, the total area ratio of ferrite and bainitic ferrite needs to be 75% or less. For better ductility, the total area ratio of ferrite and bainitic ferrite is preferably 35% or more and 70% or less. Bainitic ferrite is effective in ensuring better hole expansion formability and better fatigue properties since, as described above, it forms adjacent to ferrite and retained austenite/martensite and has the effect of reducing the difference in hardness between ferrite and retained austenite and between ferrite and martensite to suppress the occurrence of cracking during hole expansion test and of fatigue cracking during fatigue test. Therefore, the area ratio of bainitic ferrite is preferably 5% or more. On the other hand, to secure stable strength, the area ratio of bainitic ferrite is preferably 25% or less.

As used herein, the term “bainitic ferrite” means such ferrite that is produced during the process of annealing at a temperature of 740° C. or higher and 840° C. or lower, followed by cooling to and holding at a temperature of 600° C. or lower, and that has a high dislocation density as compared to normal ferrite.

While the main example of ferrite is acicular ferrite, ferrite may include polygonal ferrite and non-recrystallized ferrite. To ensure good ductility, however, it is preferred that the area ratio of polygonal ferrite is 20% or less and the area ratio of non-recrystallized ferrite is 5% or less. The area ratios of polygonal ferrite and of non-recrystallized ferrite may be 0%.

The area ratios of ferrite and bainitic ferrite can be determined by polishing a cross section of a steel sheet taken in the sheet thickness direction to be parallel to the rolling direction (L-cross section), etching the cross section with 3 vol. % nital, and averaging the results from observing ten locations at 2000 times magnification under an SEM (scanning electron microscope) at a position of sheet thickness×¼ (a position at a depth of one-fourth of the sheet thickness from the steel sheet surface) and calculating the area ratios of ferrite and bainitic ferrite for the ten locations with Image-Pro, available from Media Cybernetics, Inc., using the structure micrographs imaged with the SEM.

In the structure micrographs, ferrite and bainitic ferrite appear as a gray structure (base steel structure), while retained austenite and martensite as a white structure.

Identification of ferrite and bainitic ferrite is made by EBSD (Electron Back Scatter Diffraction) measurement. Specifically, a crystal grain (phase) that includes a sub-boundary with a grain boundary angle of smaller than 15° is identified as bainitic ferrite, for which the area ratio is calculated and used as the area ratio of bainitic ferrite. The area ratio of ferrite can be calculated by subtracting the area ratio of bainitic ferrite from the area ratio of the above-described gray structure.

Area Ratio of Tempered Martensite: 5% or More and 15% or Less

To ensure good hole expansion formability (stretch flangeability), the area ratio of tempered martensite needs to be 5% or more. For better hole expansion formability (stretch flangeability), it is preferred that the area ratio of tempered martensite is 8% or more. If the area ratio of tempered martensite exceeds 15%, however, it becomes difficult to obtain a sufficient amount of retained austenite. This results in a difficulty in obtaining good ductility and balancing strength and ductility. Therefore, the area ratio of tempered martensite needs to be 15% or less.

Here, tempered martensite can be identified by determining whether cementite or retained austenite is included in martensite (tempered martensite is martensite containing cementite or retained austenite). The area ratio of tempered martensite can be determined by polishing an L-cross section of a steel sheet, etching the cross section with 3 vol. % nital, and averaging the results from observing ten locations at 2000 times magnification under an SEM (scanning electron microscope) at a position of sheet thickness×¼ and calculating the area ratios of ferrite and bainitic ferrite for the ten locations with Image-Pro, available from Media Cybernetics, Inc., using the structure micrographs imaged with the SEM.

Volume Fraction of Retained Austenite: 8% or More

To ensure good ductility and balance strength and ductility, the volume fraction of retained austenite needs to be 8% or more. For obtaining better ductility and achieving a better balance between strength and ductility, it is preferred that the volume fraction of retained austenite is 10% or more. No upper limit is particularly placed on the volume fraction of retained austenite, yet it is around 35%. The volume fraction of retained austenite is calculated by determining the x-ray diffraction intensity of a plane of sheet thickness×¼, which is exposed by polishing the steel sheet surface to a depth of one-fourth of the sheet thickness. Using an incident x-ray beam of MoKα, the intensity ratio of the peak integrated intensity of the {111}, {200}, {220}, and {311} planes of retained austenite to the peak integrated intensity of the {110}, {200}, and {211} planes of ferrite is calculated for all of the twelve combinations, the results are averaged, and the average is used as the volume fraction of retained austenite.

Mean Grain Size of Retained Austenite: 2 μm or Less

Refinement of retained austenite grains contributes to improving the ductility and fatigue properties of the steel sheet. Accordingly, to ensure good ductility and fatigue properties, retained austenite needs to have a mean grain size of 2 μm or less. For better ductility and fatigue properties, it is preferred that retained austenite has a mean grain size of 1.5 μm or less. No lower limit is particularly placed on the mean grain size, yet it is around 0.1 μm. The mean grain size of retained austenite can be determined by averaging the results from observing twenty locations at 15000 times magnification under a TEM (transmission electron microscope) and averaging the equivalent circular diameters calculated from the areas of retained austenite grains identified with Image-Pro, as mentioned above, using the structure micrographs imaged with the TEM.

Mean Free Path of Bainitic Ferrite: 3 μm or Less

The mean free path of bainitic ferrite is very important. Specifically, bainitic ferrite forms in the process of cooling to and holding at a temperature of 600° C. or lower following the annealing in a temperature range of 740° C. to 840° C. In this respect, bainitic ferrite forms adjacent to ferrite and retained austenite, and has the effect of reducing the difference in hardness between ferrite and retained austenite to suppress the occurrence of fatigue cracking and propagation of cracking. It is thus more advantageous if bainitic ferrite is densely distributed, in other words, if bainitic ferrite has a small mean free path. To ensure good fatigue properties, bainitic ferrite needs to have a mean free path of 3 μm or less. For better fatigue properties, it is preferred that bainitic ferrite has a mean free path of 2.5 μm or less. No lower limit is particularly placed on the mean free path, yet it is around 0.5 μm.

The mean free path (L_(BF)) of bainitic ferrite can be calculated by:

$L_{BF} = {{\frac{d_{BF}}{2}\left( \frac{4\pi}{3\; f} \right)^{\frac{1}{3}}} - d_{BF}}$

L_(BF): mean free path of bainitic ferrite (μm)

d_(BF): mean grain size of bainitic ferrite (μm)

f: area ratio of bainitic ferrite (%)÷100

The mean grain size of bainitic ferrite can be determined by averaging the areas of grains by dividing the area of bainitic ferrite in the measured region calculated by EBSD (Electron Back Scatter Diffraction) measurement by the number of bainitic ferrite grains in the measured region to identify an equivalent circle diameter.

In addition to ferrite and bainitic ferrite, tempered martensite, and retained austenite, the microstructures according to the disclosure may include carbides such as martensite, pearlite, cementite, and the like, as well as other microstructures well known as steel sheet microstructures. Any microstructure that has an area ratio of 15% or less may be used without detracting from the effect of the disclosure.

Examples

Steels having the chemical compositions presented in Table 1, each with the balance consisting of Fe and incidental impurities, were prepared by steelmaking in a converter and formed into slabs by continuous casting. The steel slabs thus obtained were heated under the conditions presented in Table 2, and subjected to hot rolling, followed by pickling treatment. For Steel Nos. 1, 3-6, 8, 9, 12, 14, 16-19, 21, 24, 26, 29, 31, 33, 35, 37, 38, 40, 42, 43, 47, 50, 51, 53, 56, and 60 presented in Table 2, cold rolling was not performed, and annealing treatment was conducted under the conditions presented in Table 2 to produce high-strength hot-rolled steel sheets (HR). For Steel Nos. 2, 7, 10, 11, 13, 15, 20, 22, 23, 25, 27, 28, 30, 32, 34, 36, 39, 41, 44-46, 48, 49, 52, 54, 55, 57-59, and 61 presented in Table 2, cold rolling was performed, and annealing treatment was conducted under the conditions presented in Table 2 to produce high-strength cold-rolled steel sheets (CR). Moreover, some were subjected to galvanizing treatment to obtain hot-dip galvanized steel sheets (GI), galvannealed steel sheets (GA), and electrogalvanized steel sheets (EG).

Used as hot-dip galvanizing baths were a zinc bath containing 0.19 mass % of Al for GI and a zinc bath containing 0.14 mass % of Al for GA, in each case the bath temperature was 465° C. The coating weight per side was 45 g/m² (in the case of both-sided coating), and the Fe concentration in the coated layer of each hot-dip galvannealed steel sheet (GA) was 9 mass % or more and 12 mass % or less. The Ac₁ transformation temperature (° C.) presented in Table 1 was calculated by:

Ac₁ transformation temperature (° C.)=751−16×(% C)+11×(% Si)−28×(% Mn)−5.5×(% Cu)+13×(% Cr)

-   -   Where (% X) represents content (in mass %) of an element X in         steel.

TABLE 1 Ac₁ transformation Chemical composition (mass %) temperature Steel ID C Si Mn P S N B Ti Al Nb Cr Cu Sb Sn Ta Ca Mg REM (° C.) Remarks A 0.112 1.62 2.38 0.021 0.0020 0.0032 — — — — — — — — — — — — 700 Conforming steel B 0.182 1.24 2.11 0.019 0.0019 0.0030 — — — — — — — — — — — — 703 Conforming steel C 0.211 1.28 2.14 0.014 0.0018 0.0032 — — — — — — — — — — — — 705 Conforming steel D 0.232 0.73 2.34 0.025 0.0022 0.0030 — — — — — — — — — — — — 690 Conforming steel E 0.224 1.02 2.13 0.029 0.0016 0.0032 — — — — — — — — — — — — 702 Conforming steel F 0.218 1.48 2.09 0.016 0.0024 0.0033 — — — — — — — — — — — — 709 Conforming steel G 0.228 1.55 2.16 0.018 0.0019 0.0034 — — — — — — — — — — — — 704 Conforming steel H 0.200 1.48 2.34 0.022 0.0021 0.0030 — — — — — — — — — — — — 699 Conforming steel I 0.182 1.39 2.86 0.028 0.0019 0.0029 — — — — — — — — — — — — 683 Conforming steel J 0.064 1.51 2.89 0.027 0.0018 0.0028 — — — — — — — — — — — — 686 Comparative steel K 0.232 0.24 2.78 0.023 0.0021 0.0030 — — — — — — — — — — — — 672 Comparative steel L 0.213 1.43 1.72 0.028 0.0028 0.0028 — — — — — — — — — — — — 715 Comparative steel M 0.202 1.34 2.22 0.018 0.0024 0.0034 — — 0.380 — — — — — — — — — 700 Conforming steel N 0.198 1.22 2.11 0.031 0.0022 0.0031 — 0.034 — — — — — — — — — — 707 Conforming steel O 0.188 1.24 2.25 0.016 0.0026 0.0032 — — — 0.041 — — — — — — — — 709 Conforming steel P 0.234 1.48 2.41 0.028 0.0018 0.0030 — — — — 0.22 — — — — — — — 712 Conforming steel Q 0.203 1.46 2.21 0.015 0.0024 0.0029 — — — — — 0.25 — — — — — — 701 Conforming steel R 0.221 1.49 2.18 0.024 0.0019 0.0033 — — — — — — 0.0051 — — — — — 703 Conforming steel S 0.187 1.56 2.34 0.019 0.0028 0.0034 — — — — — — — 0.0046 — — — — 710 Conforming steel T 0.189 1.45 2.03 0.024 0.0018 0.0029 — — — — — — — — 0.0039 — — — 707 Conforming steel U 0.199 1.32 2.09 0.025 0.0017 0.0044 — — — 0.041 — — 0.0060 — — — — — 704 Conforming steel V 0.202 1.38 2.12 0.018 0.0026 0.0036 — — — 0.020 — — — 0.0062 — — — — 704 Conforming steel W 0.211 1.46 2.28 0.028 0.0025 0.0042 — — — 0.034 — — — — 0.0055 — — — 709 Conforming steel X 0.213 1.24 2.42 0.019 0.0022 0.0044 — — — — — — — — — 0.0024 — — 707 Conforming steel Y 0.197 1.44 2.21 0.024 0.0019 0.0036 — — — — — — — — — — 0.0016 — 702 Conforming steel Z 0.198 1.63 2.09 0.020 0.0017 0.0032 — — — — — — — — — — — 0.0020 707 Conforming steel AA 0.178 1.42 2.30 0.012 0.0015 0.0033 0.0019 0.021 — — — — — — — — — — 699 Conforming steel AB 0.252 1.01 2.11 0.018 0.0022 0.0041 0.0012 0.018 — — — — — — — — — — 699 Conforming steel AC 0.181 1.49 2.61 0.019 0.0021 0.0032 0.0011 0.023 — — — — — — — — — — 691 Conforming steel AD 0.143 0.98 2.79 0.012 0.0016 0.0035 0.0026 0.032 — — — — — — — — — — 681 Conforming steel AE 0.112 1.39 2.33 0.021 0.0024 0.0039 0.0021 0.026 — — — — — — — — — — 699 Conforming steel AF 0.109 1.21 2.18 0.019 0.0021 0.0033 — — — — — — — — — — — — 702 Conforming steel AG 0.115 1.30 2.86 0.028 0.0019 0.0034 0.0030 0.018 — — — — — — — — — — 683 Conforming steel AH 0.113 0.89 2.12 0.015 0.0018 0.0030 — — — — — — — — — — — — 700 Conforming steel AI 0.126 0.97 2.83 0.024 0.0021 0.0029 0.0025 0.030 — — — — — — — — — — 680 Conforming steel AJ 0.121 2.30 2.89 0.019 0.0028 0.0028 — — — — — — — — — — — — 693 Conforming steel AK 0.308 1.22 2.10 0.024 0.0024 0.0030 0.0015 0.028 — — — — — — — — — — 701 Conforming steel AL 0.295 1.44 2.41 0.022 0.0022 0.0028 — — — — — — — — — — — — 695 Conforming steel AM 0.293 1.22 2.69 0.021 0.0026 0.0034 0.0035 0.015 — — — — — — — — — — 684 Conforming steel AN 0.131 1.33 2.38 0.022 0.0018 0.0037 0.0018 0.060 — — — — — — — — — — 697 Conforming steel AO 0.177 1.47 2.79 0.006 0.0024 0.0032 — — — — — — — — — — — — 686 Conforming steel AP 0.190 1.40 2.68 0.017 0.0007 0.0041 0.0020 0.025 — — — — — — — — — — 688 Conforming steel AQ 0.228 1.32 2.47 0.007 0.0006 0.0040 — — — — — — — — — — — — 693 Conforming steel Underlined if outside of the appropriate range.

TABLE 2 Hot-rolling Reheating treatment conditions Cold-rolling Annealing treatment conditions conditions Slab Finisher Mean conditions Annealing Mean Cooling Reheating heating delivery coiling Rolling Annealing holding cooling stop Reheating holding Steel temp. temp. temp. reduction temp. time rate temp. temp. time No. ID (° C.) (° C.) (° C.) (%) (° C.) (s) (° C./s) (° C.) (° C.) (s) Type* Remarks  1 A 1250  910 400 cold rolling not 770  120 17 220 400 190 HR Example performed  2 B 1260  890 440 13.0 790  150 20 190 500 340 GI Example  3 C 1230  870 410 cold rolling not 780  140 22 200 420 210 HR Example performed  4 C  890  900 400 cold rolling not 810  200 15 230 430 150 HR Comparative performed example  5 C 1420  910 420 cold rolling not 800  240 16 200 450 130 HR Comparative performed example  6 C 1220  640 380 cold rolling not 810  280 17 190 390 210 HR Comparative performed example  7 C 1230 1120 490  6.0 800  180 17 220 400 290 CR Comparative example  8 C 1240  910 120 cold rolling not 790  300 18 240 400 210 GI Comparative performed example  9 C 1260  890 630 cold rolling not 790  250 22 250 420 230 HR Comparative performed example 10 C 1230  900 420 46.2 820  200 17 280 500 240 CR Comparative example 11 C 1230  920 450 13.0 660  280 15 240 480 180 EG Comparative example 12 C 1220  860 470 cold rolling not 900  100 16 200 490 210 HR Comparative performed example 13 C 1240  870 460  5.3 780   5 17 170 460 290 CR Comparative example 14 C 1250  900 480 cold rolling not 790 1200 17 300 440 260 HR Comparative performed example 15 C 1260  910 500  8.7 800  180 72 260 420 190 EG Comparative example 16 C 1250  900 480 cold rolling not 810  220 17  70 400 160 GI Comparative performed example 17 C 1230  860 460 cold rolling not 800  240 15 550 450 170 HR Comparative performed example 18 C 1240  900 450 cold rolling not 810  180 14 220 270 150 HR Comparative performed example 19 C 1200  870 420 cold rolling not 820  150 12 200 620 200 HR Comparative performed example 20 C 1230  890 400  8.0 810  300 18 230 420  5 GA Comparative example 21 C 1240  880 450 cold rolling not 790  180 20 220 500 950 GI Example performed 22 D 1220  890 460 11.1 770  180 24 200 480 480 CR Example 23 E 1230  900 420 11.1 790  200 24 240 380 260 CR Example 24 F 1240  910 480 cold rolling not 760  240 22 220 400 270 GA Example performed 25 G 1230  880 500  6.3 790  190 20 190 460 190 CR Example 26 H 1220  860 470 cold rolling not 760  150 22 200 450 170 EG Example performed 27 I 1210  880 490  8.7 820  100 19 220 480 150 CR Example 28 J 1200  860 500  8.0 760  180 22 240 430 190 CR Comparative example 29 K 1230  890 470 cold rolling not 820  150 17 230 400 510 EG Comparative performed example 30 L 1230  890 460  4.3 800  170 16 210 420 200 CR Comparative example 31 M 1250  900 420 cold rolling not 820  200 18 200 480 450 GI Example performed 32 N 1240  890 450  5.3 750  90 16 210 500 510 CR Example 33 O 1240  880 460 cold rolling not 780  120 27 220 450 180 HR Example performed 34 P 1250  860 400  5.6 790  180 26 240 410 520 CR Example 35 Q 1230  890 440 cold rolling not 800  80 17 190 400 400 EG Example performed 36 R 1220  860 400  5.3 800  160 28 200 460 180 GA Example 37 S 1230  910 380 cold rolling not 790  200 17 230 420 190 GI Example performed 38 T 1220  880 410 cold rolling not 810  240 17 240 410 380 EG Example performed 39 U 1230  880 400  5.3 790  160 16 200 400 540 GI Example 40 V 1240  890 420 cold rolling not 800  280 15 190 450 250 HR Example performed 41 W 1220  880 400  8.0 780  200 16 180 420 180 EG Example 42 X 1230  910 350 cold rolling not 810  90 22 260 400 200 HR Example performed 43 Y 1230  870 380 cold rolling not 770  150 20 240 460 180 GI Example performed 44 Z 1210  860 400  5.3 800  200 20 200 450 190 CR Example 45 AA 1250  900 450 11.1 790  200 15 200 410 200 CR Example 46 AB 1220  910 480  9.1 800  180 14 210 430 180 GA Example 47 AC 1240  870 490 cold rolling not 780  250 13 180 410 200 HR Example performed 48 AD 1230  880 480 10.0 810  200 16 230 400 150 GI Example 49 AE 1250  900 400 11.1 820  250 14 200 410 220 CR Example 50 AF 1240  880 440 cold rolling not 790  180 22 240 380 180 HR Example performed 51 AG 1210  890 400 cold rolling not 800  200 18 220 400 150 HR Example performed 52 AH 1200  900 380 12.5 820  200 22 210 460 200 GA Example 53 AI 1230  910 410 cold rolling not 790  250 19 200 450 150 HR Example performed 54 AJ 1230  880 400 13.3 830  230 21 200 450 190 EG Example 55 AK 1240  880 420  6.3 790  160 17 220 390 510 CR Example 56 AL 1220  890 400 cold rolling not 760  300 16 210 400 200 HR Example performed 57 AM 1230  880 350  7.7 780  170 17 190 400 450 CR Example 58 AN 1230  910 420  6.7 800  250 16 270 420 510 CR Example 59 AO 1210  860 380  6.7 820  90 26 190 500 190 CR Example 60 AP 1230  880 400 cold rolling not 810  100 17 220 480 410 HR Example performed 61 AQ 1250  900 420  6.7 810  200 18 210 400 350 GI Example Underlined if outside of the appropriate range. *HR: Hot-rolled steel sheets (uncoated), CR: Cold-rolled steel sheets (uncoated), GI: hot-dip galvanized steel sheets (alloying treatment not performed on galvanized layers), GA: galvannealed steel sheets, EG: electrogalvanized steel sheets

The high-strength hot-rolled steel sheets (HR), high-strength cold-rolled steel sheets (CR), hot-dip galvanizing steel sheets (GI), galvannealed steel sheets (GA), and electrogalvanized steel sheets (EG) thus obtained were subjected to structure observation, tensile test, hole expansion test, and fatigue test.

In this case, tensile test was performed in accordance with JIS Z 2241 (2011) to measure TS (tensile strength) and EL (total elongation), using JIS No. 5 test pieces that were sampled such that the longitudinal direction of each test piece coincides with a direction perpendicular to the rolling direction of the steel sheet (the C direction). In this case, TS and EL were determined to be good when EL≧34% for TS 780 MPa grade, EL≧27% for TS 980 MPa grade, and EL≧23% for TS 1180 MPa grade, and TS×EL≧27000 MPa·%.

Further, hole expansion test was performed in accordance with JIS Z 2256 (2010). Each of the steel sheets thus obtained was cut to a sample size of 100 mm×100 mm, and a hole with a diameter of 10 mm was drilled through each sample with clearance 12%±1%. Subsequently, each steel sheet was clamped into a die having an inner diameter of 75 mm with a blank holding force of 8 tons (7.845 kN). In this state, a conical punch of 60° was pushed into the hole, and the hole diameter at the time of occurrence of cracking (hole diameter at crack initiation limit) was measured. Based on the hole diameter thus measured, the maximum hole expansion ratio λ (%) was calculated by the following equation to evaluate hole expansion formability:

maximum hole expansion ratio λ(%)={(D _(f) −D ₀)/D ₀}×100

-   -   Where D_(f) is a hole diameter at the time of occurrence of         cracking (mm) and D₀ is an initial hole diameter (mm).         In this case, TS and EL were determined to be good when λ≧40%         for TS 780 MPa grade, λ≧30% for TS 980 MPa grade, and λ≧20% TS         1180 MPa grade.

Moreover, in fatigue test, sampling was performed such that the longitudinal direction of each fatigue test piece coincides with a direction perpendicular to the rolling direction of the steel sheet, and plane bending fatigue test was conducted under the completely reversed (stress ratio: −1) condition and at the frequency of 20 Hz in accordance with JIS Z 2275 (1978). In the completely reversed plane bending fatigue test, the stress at which no fracture was observed after 10⁷ cycles was measured and used as fatigue limit strength.

Fatigue limit strength was divided by tensile strength TS to calculate a fatigue ratio. In this case, the fatigue property was determined to be good when fatigue limit strength ≧400 MPa and fatigue ratio ≧0.40.

Additionally, during the manufacture of steel sheets, measurements were made of productivity, sheet passage ability during hot rolling and cold rolling, and surface characteristics of each steel sheet obtained after final annealing (hereinafter also referred to as a “final-annealed sheet”).

In this case, productivity was evaluated according to the lead time costs, including:

-   -   (1) malformation of a hot-rolled steel sheet occurred;     -   (2) a hot-rolled steel sheet requires straightening before         proceeding to the subsequent steps;     -   (3) a prolonged annealing treatment holding time; and     -   (4) a prolonged austemper holding time (a prolonged holding time         in a reheating temperature range in annealing treatment).         The productivity was determined to be “high” when none of (1)         to (4) applied, “middle” when only (4) applied, and “low” when         any of (1) to (3) applied.

The sheet passage ability during hot rolling was determined to be low when the risk of trouble during rolling increased with increasing rolling load. Similarly, the sheet passage ability during cold rolling was determined to be low when the risk of trouble during rolling increased with increasing rolling load.

Furthermore, the surface characteristics of each final-annealed sheet were determined to be poor when defects such as blow hole generation and segregation on the surface layer of the slab could not be scaled-off, cracks and irregularities on the steel sheet surface increased, and a smooth steel sheet surface could not be obtained. The surface characteristics were also determined to be poor when the amount of oxides (scales) generated suddenly increased, the interface between the steel substrate and oxides was roughened, and the surface quality after pickling and cold rolling degraded, or when some hot-rolling scales remained after pickling.

Structure observation was performed following the above-described procedure. The evaluation results are shown in Tables 3 and 4.

TABLE 3 Steel structure Mean Mean Area Area Volume grain free Sheet ratio of ratio fraction size path Steel thickness F + BF of TM of RA of RA of BF Balance No. ID (mm) (%) (%) (%) (μm) (μm) structure Remarks  1 A 2.3 69.1  9.2 11.9 0.6 1.8 M + P + θ Example  2 B 2.0 68.4  9.8 10.2 0.7 1.7 M + P + θ Example  3 C 2.3 67.8 11.1 12.2 0.7 2.0 M + P + θ Example  4 C 2.9 63.6 10.4 17.1 1.4 2.1 M + P + θ Comparative example  5 C 2.5 62.2 11.1 16.8 1.3 2.4 M + P + θ Comparative example  6 C 2.5 59.2  9.7  6.8 0.6 5.6 M + P + θ Comparative example  7 C 2.3 65.7 10.6 12.5 2.9 2.2 M + P + θ Comparative example  8 C 1.9 64.9 12.2 15.4 1.4 2.4 M + P + θ Comparative example  9 C 1.4 70.6  8.9  3.8 0.5 2.5 M + P + θ Comparative example 10 C 1.4 66.9  8.6  9.1 3.8 5.2 M + P + θ Comparative example 11 C 2.0 64.2  1.2  5.7 3.0 2.6 M + P + θ Comparative example 12 C 2.1 66.4 23.4  9.1 3.1 2.7 M + P + θ Comparative example 13 C 1.8 67.6  5.6  6.7 3.4 2.4 M + P + θ Comparative example 14 C 1.7 85.6  7.9  3.2 1.6 2.1 M + P + θ Comparative example 15 C 2.1 54.8 26.0 11.0 1.7 2.2 M + P + θ Comparative example 16 C 1.7 63.1 31.4  3.3 3.4 2.2 M + P + θ Comparative example 17 C 2.3 64.6  0.6  2.9 0.5 2.3 M + P + θ Comparative example 18 C 1.8 46.9 37.8  2.4 0.6 1.8 M + P + θ Comparative example 19 C 2.1 48.2 10.6  4.2 0.7 2.2 M + P + θ Comparative example 20 C 2.3 63.7  3.1  3.5 0.6 2.4 M + P + θ Comparative example 21 C 1.9 66.6  9.6 14.4 0.8 2.5 M + P + θ Example 22 D 1.6 59.9 12.1 14.5 1.1 1.9 M + P + θ Example 23 E 1.6 66.6 11.6 11.4 1.2 1.8 M + P + θ Example 24 F 1.9 67.4 10.8 10.9 0.9 1.7 M + P + θ Example 25 G 1.5 68.4  9.2 11.4 0.7 1.9 M + P + θ Example 26 H 1.8 66.5  8.4 12.8 0.9 1.5 M + P + θ Example 27 I 2.1 58.2 12.8 15.6 0.8 2.0 M + P + θ Example 28 J 2.3 83.3  5.5  2.1 0.3 2.3 M + P + θ Comparative example 29 K 2.5 48.4 26.2  3.5 0.6 2.1 M + P + θ Comparative example 30 L 2.2 81.7  0.5  4.6 0.7 2.4 M + P + θ Comparative example 31 M 2.5 65.4 11.4 11.1 0.7 1.7 M + P + θ Example 32 N 1.8 66.5 10.9 11.9 0.9 1.5 M + P + θ Example 33 O 1.7 64.4  9.7 12.8 1.1 1.2 M + P + θ Example 34 P 1.7 67.7  9.9 11.4 0.9 1.6 M + P + θ Example 35 Q 2.4 64.5 10.6 11.4 1.0 1.1 M + P + θ Example 36 R 1.8 68.2 11.2  9.1 0.7 1.8 M + P + θ Example 37 S 2.7 71.7  8.9  9.6 0.6 2.0 M + P + θ Example 38 T 2.5 69.7  9.7 10.1 0.5 1.2 M + P + θ Example 39 U 1.8 67.6 10.4 11.4 0.7 1.5 M + P + θ Example 40 V 2.5 65.4 10.1 12.5 0.5 1.8 M + P + θ Example 41 W 2.3 63.0 11.8 13.6 0.6 1.1 M + P + θ Example 42 X 1.9 68.4  9.4 11.6 0.7 0.9 M + P + θ Example 43 Y 2.5 66.1 10.6 12.8 0.9 1.5 M + P + θ Example 44 Z 1.8 67.4  9.7 12.5 0.9 1.6 M + P + θ Example 45 AA 1.6 68.3 11.2 11.1 0.8 1.6 M + P + θ Example 46 AB 2.0 66.9 12.4 13.2 0.9 1.7 M + P + θ Example 47 AC 2.2 65.1 12.9 14.8 1.1 2.1 M + P + θ Example 48 AD 1.8 66.2 10.8 12.1 0.7 1.9 M + P + θ Example 49 AE 1.6 68.9  9.2 10.9 0.6 1.6 M + P + θ Example 50 AF 2.0 69.2 12.1 12.5 1.3 2.2 M + P + θ Example 51 AG 1.8 68.9 11.6 11.4 1.4 2.3 M + P + θ Example 52 AH 1.4 69.1 10.8 10.9 1.0 1.8 M + P + θ Example 53 AI 1.8 67.5 12.2 11.4 0.9 2.2 M + P + θ Example 54 AJ 1.3 66.6 11.4 13.8 0.7 2.4 M + P + θ Example 55 AK 1.5 62.9 12.8 15.6 0.7 2.5 M + P + θ Example 56 AL 2.0 61.9 11.9 22.5 0.9 1.9 M + P + θ Example 57 AM 1.2 56.7 10.8 23.5 0.9 1.8 M + P + θ Example 58 AN 1.4 64.1  9.2 18.3 0.7 1.7 M + P + θ Example 59 AO 1.4 61.3 11.6 21.3 0.8 1.9 M + P + θ Example 60 AP 1.8 59.9 10.7 22.1 1.0 1.9 M + P + θ Example 61 AQ 1.4 57.7 10.4 24.9 1.1 1.8 M + P + θ Example Underlined if outside of the appropriate range. F: ferrite, BF: bainitic ferrite, RA: retained austenite, M: martensite, TM: tempered martensite, P: peadite, θ: cementite

TABLE 4 Sheet Sheet Hole Fatigue test results passage passage expansion Fatigue ability ability Surface Tensile test results test results limit during during chamcteristics TS EL TS × EL λ strength Fatigue hot cold of final- No. (MPa) (%) (MPa · %) (%) (MPa) ratio Productivity rolling rolling annealed sheet Remarks  1 794 40.1 31839 68 450 0.57 High High — Good Example  2 910 37.1 33761 52 460 0.51 High High High Good Example  3 1008 33.5 33768 42 470 0.47 High High — Good Example  4 1028 27.8 28578 35 410 0.40 Low Low — Fairly poor Comparative example  5 1034 27.2 28125 33 410 0.40 Low Low — Fairly poor Comparative example  6 1235 12.4 15314 26 500 0.40 Low Low — Fairly poor Comparative example  7 1012 18.9 19127 34 410 0.41 Low High Low Poor Comparative example  8 942 28.1 26470 42 400 0.42 Low High — Good Comparative example  9 679 34.1 23154 50 280 0.41 High High — Good Comparative example 10 1044 15.8 16495 26 290 0.28 High High High Good Comparative example 11 1189 16.2 19262 16 480 0.40 High High High Good Comparative example 12 1022 18.4 18805 38 410 0.40 Low High — Good Comparative example 13 1279 14.8 18929 24 520 0.41 High High High Good Comparative example 14 682 26.9 18346 45 290 0.43 Low High — Good Comparative example 15 1289 8.9 11472 24 510 0.40 High High High Good Comparative example 16 802 20.5 16441 52 340 0.42 High High — Good Comparative example 17 1030 27.6 28428 24 480 0.47 High High — Good Comparative example 18 716 24.5 17542 53 300 0.42 High High — Good Comparative example 19 1199 14.7 17625 21 480 0.40 High High — Good Comparative example 20 1088 14.2 15450 14 490 0.45 High High High Good Comparative example 21 1011 28.9 29218 35 430 0.43 Middle High — Good Example 22 1122 30.1 33772 36 470 0.42 High High High Good Example 23 1000 33.4 33400 38 430 0.43 High High High Good Example 24 1041 30.8 32063 35 440 0.42 High High — Good Example 25 984 34.5 33948 41 420 0.43 High High High Good Example 26 1008 33.1 33365 37 440 0.44 High High — Good Example 27 1211 27.8 33666 27 510 0.42 High High High Good Example 28 678 25.8 17492 68 310 0.46 High High High Good Comparative example 29 1245 10.9 13571 14 520 0.42 High High — Good Comparative example 30 679 26.9 18265 40 320 0.47 High High High Good Comparative example 31 1056 30.1 31786 45 450 0.43 High High — Good Example 32 1047 29.8 31201 40 440 0.42 High High High Good Example 33 1070 28.4 30388 36 470 0.44 High High — Good Example 34 1004 32.9 33032 39 480 0.48 High High High Good Example 35 1007 32.4 32627 46 450 0.45 High High — Good Example 36 1004 33.9 34036 41 430 0.43 High High High Good Example 37 827 39.1 32336 51 410 0.50 High High — Good Example 38 908 35.5 32234 53 420 0.46 High High — Good Example 39 1001 33.6 33634 42 430 0.43 High High High Good Example 40 1033 32.0 33056 39 460 0.45 High High — Good Example 41 1107 28.9 31992 40 450 0.41 High High High Good Example 42 1002 33.7 33767 39 480 0.48 High High — Good Example 43 1039 32.6 33871 38 440 0.42 High High — Good Example 44 1026 32.8 33653 40 500 0.49 High High High Good Example 45 989 32.2 31846 56 450 0.46 High High High Good Example 46 1036 30.8 31909 62 460 0.44 High High High Good Example 47 1198 29.2 34982 48 510 0.43 High High — Good Example 48 996 32.1 31972 54 450 0.45 High High High Good Example 49 810 37.8 30618 61 440 0.54 High High High Good Example 50 822 34.1 28030 48 430 0.52 High High — Good Example 51 1014 27.9 28291 39 490 0.48 High High — Good Example 52 797 34.9 27815 45 400 0.50 High High High Good Example 53 1002 28.8 28858 38 470 0.47 High High — Good Example 54 1189 24.4 29012 31 520 0.44 High High High Good Example 55 1092 30.7 33524 37 490 0.45 High High High Good Example 56 1111 29.9 33219 33 520 0.47 High High — Good Example 57 1239 28.2 34940 28 560 0.45 High High High Good Example 58 985 30.6 30141 41 480 0.49 High High High Good Example 59 1134 28.7 32546 37 500 0.44 High High High Good Example 60 1122 28.2 31640 39 520 0.46 High High — Good Example 61 1086 31.9 34643 45 500 0.46 High High High Good Example

It can be seen that each of our examples has TS of 780 MPa or more, and the present disclosure enables manufacture of high-strength steel sheets with high productivity that are excellent not only in ductility but also in hole expansion formability (stretch flangeability) and fatigue properties. It can also be appreciated that each of our examples exhibits excellent sheet passage ability during hot rolling and cold rolling, as well as excellent surface characteristics of the final-annealed sheet.

In contrast, comparative examples are inferior in terms of one or more of tensile strength, ductility, balance between strength and ductility, hole expansion formability (stretch flangeability), fatigue properties, and productivity. 

1. A method for manufacturing a high-strength steel sheet, the method comprising: preparing a steel slab containing, in mass %, C: 0.10% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and the balance consisting of Fe and incidental impurities; subjecting the steel slab to hot rolling by heating the steel slab to a temperature of 1100° C. or higher and 1300° C. or lower, hot rolling the steel slab with a finisher delivery temperature of 800° C. or higher and 1000° C. or lower to form a hot-rolled steel sheet, and coiling the hot-rolled steel sheet at a mean coiling temperature of 200° C. or higher and 500° C. or lower; subjecting the hot-rolled steel sheet to pickling treatment; subjecting the hot-rolled steel sheet to annealing by retaining the hot-rolled steel sheet at a temperature of 740° C. or higher and 840° C. or lower for 10 s or more and 900 s or less, and then cooling the hot-rolled steel sheet at a mean cooling rate of 5° C./s or higher and 30° C./s or lower to a cooling stop temperature of 150° C. or higher and 350° C. or lower; and subjecting the hot-rolled steel sheet to reheating treatment by reheating the hot-rolled steel sheet to a reheating temperature of higher than 350° C. and 550° C. or lower, and retaining the hot-rolled steel sheet at the reheating temperature for 10 s or more.
 2. The method for manufacturing a high-strength steel sheet according to claim 1, the method further comprising prior to the annealing, cold rolling the hot-rolled steel sheet at a rolling reduction of less than 30% to form a cold-rolled steel sheet, wherein in the annealing, the cold-rolled steel sheet is retained at a temperature of 740° C. or higher and 840° C. or lower for 10 s or more and 900 s or less, and cooled at a mean cooling rate of 5° C./s or higher and 30° C./s or lower to a cooling stop temperature of 150° C. or higher and 350° C. or lower, and in the reheating treatment, the cold-rolled steel sheet is reheated to a reheating temperature of higher than 350° C. and 550° C. or lower and retained at the reheating temperature for 10 s or more.
 3. The method for manufacturing a high-strength steel sheet according to claim 1, the method further comprising after the reheating treatment, subjecting the hot-rolled steel sheet or the cold-rolled steel sheet to galvanizing treatment.
 4. The method for manufacturing a high-strength steel sheet according to claim 1, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.
 5. The method for manufacturing a high-strength steel sheet according to claim 1, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 6. A high-strength steel sheet comprising: a steel chemical composition containing, in mass %, C: 0.10% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and the balance consisting of Fe and incidental impurities; and a steel structure that contains a total of 30% or more and 75% or less by area of ferrite and bainitic ferrite, 5% or more and 15% or less by area of tempered martensite, and 8% or more by volume of retained austenite, wherein the retained austenite has a mean grain size of 2 μm or less and the bainitic ferrite has a mean free path of 3 μm or less.
 7. The high-strength steel sheet according to claim 6, wherein the steel chemical composition further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.
 8. The high-strength steel sheet according to claim 6, wherein the steel chemical composition further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 9. The method for manufacturing a high-strength steel sheet according to claim 2, the method further comprising after the reheating treatment, subjecting the hot-rolled steel sheet or the cold-rolled steel sheet to galvanizing treatment.
 10. The method for manufacturing a high-strength steel sheet according to claim 2, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.
 11. The method for manufacturing a high-strength steel sheet according to claim 3, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.
 12. The method for manufacturing a high-strength steel sheet according to claim 9, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Ti: 0.005% or more and 0.100% or less and B: 0.0001% or more and 0.0050% or less.
 13. The method for manufacturing a high-strength steel sheet according to claim 2, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 14. The method for manufacturing a high-strength steel sheet according to claim 3, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 15. The method for manufacturing a high-strength steel sheet according to claim 4, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 16. The method for manufacturing a high-strength steel sheet according to claim 9, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 17. The method for manufacturing a high-strength steel sheet according to claim 10, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 18. The method for manufacturing a high-strength steel sheet according to claim 11, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 19. The method for manufacturing a high-strength steel sheet according to claim 12, wherein the steel slab further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.
 20. The high-strength steel sheet according to claim 7, wherein the steel chemical composition further contains, in mass %, at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.002% or more and 0.200% or less, Sn: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less. 